On Fri, Apr 5, 2013 at 1:01 PM, John Kerr <johneddie.k...@gmail.com> wrote:
> Today I tried some mapping with my android tablet. I was trying to trace the > outline of a building by walking around it with OSMtracker. I have not > played with my files yet because I did notice one thing while doing this and > that is I was only accurate to 7 metres. That is not very accurate. So just > a few minutes ago I downloaded Vespucci for Android and I hope to give it a > try at the same task. John, how will Vespucci increase the accuracy of the your Android device? The accuracy is based on the quality of signals received from the GPS satellites. Working close to the walls of a building will degrade the signals available, and cause possibly even more inaccuracy. Changing the software that is reading the position reported won't make the reported position more accurate. One thing you can do to decrease the inaccuracy is to get a clear unobstructed view of the sky. Moving away from the buildings will do that for you. Have a look at these images I just created... http://wiki.openstreetmap.org/wiki/File:Extension.png A visual depiction of how using a sight line along a building face can help to reduce the position ambiguity of a GPS unit. Standing close to a building blocks the GPS signal reception, degrading position accuracy even more than normal. Moving out away from the building gets a clear sky view, and that combined with the extended length of the wall can be used to reduce the error of the actual measured item. Draw lines between the extended points, and use them as guides to draw the actual building outline. http://wiki.openstreetmap.org/wiki/File:Outline1.png Image showing how using GPS positions captured by extending the sightlines along buildings can then be used to draw the actual building outline. More GPS locations are required to be captured, but GPS position ambiguity is reduced due to being clear of the building obstruction, and also due to the reduction in position error due to mathematical angular error reduction. The further from the building the GPS locations are recorded, less error is introduced into the actual building corner position. I added a bit to my Wiki page... http://wiki.openstreetmap.org/wiki/User:Ve6srv#Extending_Sightlines_to_Reduce_GPS_Error This technique will not remove all error, but can reduce the angular errors when trying to lay out a building outline. You can still have displacement errors (all measured points can be horizontally displace, ie. the whole building might be 2 metres north of where it is supposed to be), but your walls should be closer to true than what you can achieve by walking the perimeter of the building. Complex building shapes are harder to plot using this technique, but you can extrapolate some of the wall locations using this process. -- James VE6SRV _______________________________________________ Talk-ca mailing list Talk-ca@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-ca